(a) Field of the Invention
The invention is related to a slope control device and method thereof, in particularly to a slope control device capable of predicting uniform-current-sharing level and method thereof applied in a distributed power system.
(b) Description of the Prior Art
Recently, the distributed power system is increasingly applied in the communication system and the computer server. The distributed power system comprises several small capability power modules which are connected in parallel for supplying the power to the load. Compared to the central power system, the distributed power system has several advantages, including:
1. Capability of using modulized power source: When the central power system is used for supplying power, it will take much effort to redesign the power system if the customer changes the power requirement. However, because of modulized design of the distributed power system, the user can adjust the number of parallel connected power modules to keep up with the increasing load current requirement. Besides, when one of power modules of the distributed power system is shortened or breakdown, the faulty modules can be exchanged without system interruption.
2. Capability of reducing the current stress in single power module: The distributed power system can averagely distribute the output current to the paralleled power modules. Therefore, the current stress in each module can be reduced and hence the efficiency and volume of the distributed power system can be improved. In other word, the single power module can be implemented by the power units with lower rated voltage and current.
3. Efficient heat management: Generally speaking, the life time of the electronic elements reduces half when the temperature of the junctions in the electronic elements increases 10° C. The parallel connected power system ensures equal distribution of the output current and further results in uniform decentralized heat dissipation. By equally delivering the power dissipation among a large number of power components and over a greater surface area, thermal management is effectively achieved and the reliability of the system is greatly improved.
4. Capability of implementing redundancy circuit architecture: The distributed power system is easy to achieve the redundancy circuit structure in which N+1 power modules are connected in parallel. In redundant systems, one power module is at least reserved and it provides extra output load current. The benefits of such structure include reducing total power dissipation and improving the maintainability when the failure occurs. In other words, the redundancy circuit structure is very robust and is suitable for important servers or apparatus.
Currently, there are two methods applied on the distributed power system for distributing the current averagely. One is the droop method and the other is the active current-sharing method. The major difference between the droop method and the active current-sharing method is that the paralleled power modules applied the latter method need extra pinouts. These pinouts are used for enabling current sharing control circuits of paralleled power modules to communicate the information of each output current. The average current information is carried between the modules by these pinouts and hence these are so called as current-sharing bus, abbreviated CS bus. The CS bus provides a common reference to the paralleled power modules, and hence each module adjusts its output current based on the common reference. However, the droop method does not require the average current information and therefore it needs no interconnection between modules.
In conclusion, the benefits of the droop method are simple design, easy extension, needless extra connection between power modules, high modulability and high reliability. The benefits of the active current-sharing method are better load adjustability and better uniform-current-sharing ability.
FIG. 1 illustrates a schematic view of the output voltage and the output current of the droop method in accordance with the prior art. When applying the droop method on the distributed power system, the output voltage drops as the load current increases. The current sharing mechanism of this method relies on the slope of the load regulation characteristic of the parallel connected modules. In FIG. 1, even if the output voltage-current slopes of two different modules are the same, the current I1 and the current I2 differs from each other since there is minor difference between the elements of the power modules such as the reference voltage at no load condition.
As illustrated in FIG. 1, the distribution of current is more uniform when the difference between the reference voltages at no load condition is smaller. Besides, even if the difference of each reference voltage is the same, raise of the output voltage-current slopes is also contributive to improve the current derivation between two power supply modules. However, owing to the trade-off between error percentages of current sharing and output voltage variation, conventional droop technique can not provide good load regulation and current balance at the same time. The slope of the traditional droop method is limited by the maximum output voltage variation specification, and can not be raised as high as expected.
In view of the drawbacks of the prior art, the inventor of the present invention based on years of experience in the related industry to conduct extensive researches and experiments, and finally developed a slope control device capable of predicting uniform-current-sharing level and method thereof in accordance with the present invention to overcome the aforementioned drawbacks.